Guide to smartphone hardware (1/7): Processors

NVIDIA’s Tegra SoCs

Despite the fact that this SoC is indeed designed by graphics giant NVIDIA, when I look more deeply at the graphics hardware for smartphone’s you’ll discover that the Tegra line of SoCs isn’t as amazing as their desktop line of graphics cards are. It’s not to say that the SoC is bad, just that you might have been expecting a little bit more in the GPU department.

Anyway, the types of Tegra SoCs that are currently found in smartphones and tablets come from either the Tegra 2 or Tegra 3 “Kal-El” series. Both are multi-core and utilize ARM Cortex-A9 cores in their SoCs ranging from 1.0 to 1.4 GHz; both are made using a 40nm process and both use an ultra-low power (ULP) GeForce GPU as the graphics chip.

While there are similarities between the two ranges available there are some stark differences as well. For starters, the entire Tegra 2 line is dual-core whereas the Tegra 3 line is quad-core. The Tegra 3 Kal-El line uses a more powerful GPU as well, and contains some key features that the Tegra 2 SoC doesn’t have such as a companion core. However, both also can claim the feats for shipping in the first dual- and quad-core devices: the Motorola Atrix 4G and ASUS Transformer Prime respectively.

For the Tegra 2 line there are four different SoCs available: two designed for tablets and two designed for smartphones. The Tegra 250 AP20H and Tegra 250 T20 both contain 1 GHz dual-core ARM Cortex-A9 CPUs but differ in GPU clock, the faster GPU in the 250 T20 is designed for use in tablets. Then there are the 3D-capable Tegra 250 3D AP25 and Tegra 250 3D T25 which are clocked at a higher 1.2 GHz and have faster GPUs in them.

Tegra 2 SoCs lack ARM’s Advanced SIMD extension that allows the processor to execute NEON code, but as I have mentioned in the sections for other SoCs this doesn’t seem to affect a great deal. NEON allows the processor to execute MP3 playback and GSM speech at very low CPU clock cycles and also helps in the calculation of floating-point data. Again, it’s unclear what performance benefits you gain by having this capability but it mustn’t hurt to have it or NVIDIA would have left it out of Tegra 3 as well.

A look at the components of Tegra 2

According to NVIDIA’s spec sheet for the Tegra 2 SoC it seems the chipset is capable of a quite impressive range of 1080p video decoding as well as audio decoding, better than what is available on competing chips. A quick look at the block diagram reveals that, like the TI OMAP and Snapdragon SoCs, the Tegra 2 has separate processors for dealing with video and audio: in fact not just one but three.

Tegra 3 improves on Tegra 2 in many ways and beats all the competition to the first shipping quad-core SoC. Unfortunately NVIDIA had to cut corners slightly and use four Cortex-A9 cores instead of the newer Cortex-A15’s that other manufacturers are implementing in their next chipsets, but for now this shouldn’t matter.

The clock speed of these A9 cores also gets a boost from 1.2 GHz (maximum) to 1.3 GHz in a quad-core configuration, the GPU also sees a large boost and memory gets a speed and size boost as well. Tegra 3 can now also run displays that have a resolution of up to 2048x1536 (up from 1680x1050) but can still only manage two simultaneous displays where chips like the TI OMAP and Exynos can manage four and three respectively.

This is a Tegra 3 die. Notice the five cores (four full-power, one "companion") highlighted in yellow

One other cool feature of the Tegra 3 is that along with the four fully functional Cortex-A9 cores (each with NEON MPEs) there is actually a fifth Cortex-A9 “companion” core that is made using low-power transistors. The extra core runs at 500 MHz and is designed to tackle the processing of background tasks to save power, rather than having each of the four full cores run. This system is comparable to the TI OMAP 4 series’ use of Cortex-M3 cores to save power.

The future of the Tegra series lies with “Wayne”, which is said to use the newer ARM Cortex-A15 cores in either quad- or octa-core configurations. The GeForce GPU will also see a boost, but as Tegra 3 has just been launched it could be a while before we see more solid details of what this SoC will entail.

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